where are martingale increments. There are two things to note. First, the expressions above depend on the integral of the functions the interest rate. The key idea of the paper is to approximate these integrals by taking sums of the interest rates obtained at higher frequency. Second, the system would be linear in parameters if the capital rental rate rt was observed and thus could easily be estimated using usual techniques. The authors suggest to replace the unknown interest rate rt with a proxy rbt that can be obtained using its relation to the risk free interest rate and some parameter values δ0 and . Once the system is linear, usual regression-based methods can be applied